So-called spring-assisted foldable knives have been around at least since the early 1800's. These knives generally include a handle and a pivotally attached knife blade. A spring mechanism is attached between the handle and the knife blade so that the knife blade is biased to an open position. A locking mechanism is positioned within the handle portion and is designed to selectively engage the knife blade so that the blade may be held in a closed position, safely within the handle and against the action of the spring mechanism until it is needed. In use, an operator of the knife merely has to release the locking mechanism to cause the spring to force the blade to pivot to its open position. Once open, the locking mechanism typically engages the blade to hold it in its open position.
To close this prior art spring-assisted folding blade, the operator must first disengage the locking mechanism and then manually pivot the blade, against the action of the spring to its closed and again locked position. Some types of knives provide spring assist for both opening and closing the blade action.
U.S. Pat. Nos. 5,802,722 and 6,145,202 both of Onion both disclose a spring-assisted foldable knife assembly that includes a handle portion, a knife blade that is pivotally attached to the handle and operates within a knife plane. A torsion bar positioned within the handle and adjacent to the knife plane is attached between the handle and the knife blade. The torsion bar creates a “balanced” spring bias that applies either an opening force or a closing force depending on the angular position of the knife blade with respect to the handle.
In use, the operator of this type of knife pivots the blade towards its open position against the closing action of the torsion bar. When the blade is pivoted past a point of equilibrium, the spring bias of the torsion bar begins to apply an opening force to the blade and forces the blade to quickly pivot to its fully open position. To close the blade, the user first releases a locking mechanism and then forces the blade against the spring bias of the torsion bar, again until the blade passes the point of equilibrium, at which point the torsion bar pulls the blade to its closed position, safely within the handle.